1. Field of the Invention
This invention pertains to apparatus for sensing the height of a stack of sheet members and also to apparatus for controlling the height of a stack of sheet members to a predetermined value.
2. Description of the Prior Art
Apparatus for sensing the height of a stack of sheet members are used in a variety of applications including in the output section of photocopiers. Conventional stack height sensors include not only noncontact optical, pneumatic or capacitive sensors but also mechanical contact sensors. Existing optical sensors, including those utilizing reflective optics, are susceptible to contamination, variation of sensitivity due to paper type, color, curl, stack, density and location of the sheet members at positions other than at the calibrated focal point of the sensor. Also, the possibility of faulty operation due to ambient light leakage also exist in such sensing systems. Conventional capacitative and pneumatic sensors are costly and/or have problems limting their usefulness.
Conventional "mechanical feeler" type stack sensors typically utilize a rigid finger element which is indexed into and out of engagement with the top of the stack, and representative of these devices are the apparatus shown in U.S. Pat. No. 4,469,320 (Wenthe Jr.) and U.S. Pat. No. 4,589,645 (Tracy). While such devices perform adequately, the mechanical assembles used to index and reposition the sensing arm are relatively complex. The complexity, in turn, directly effects the overall cost and the reliability of the apparatus, as one skilled in the art would readily appreciate.
Also, many of the contact type stack height sensors, for example the apparatus shown in Wenthe Jr., is suitable for stacks fed from the bottom. In many applications, e.g. photocopier disc stackers, the paper or other sheet member descends onto the stack. Such applications would effectively rule out the use of certain types of conventional contact sensors and, prior to the present invention, would have required the more costly and problem-prone optical or possibly pneumatic sensors.
Hence, there still exists the need for relatively low cost but reliable stack height sensing apparatus particularly apparatus which can be readily integrated with the feeding mechanism for the sheet members, such as in the output section of photocopiers.